Not applicable.
Not applicable.
The liquefaction of natural gas at remote sites, transportation of the liquefied natural gas (LNG) to population centers, and storage and vaporization of LNG for local consumption have been successfully practiced for many years around the world. LNG production sites typically are located on land at remote sites having docking facilities for large LNG tankers which transport the LNG to end users.
Numerous process cycles have been developed for LNG production to provide the large refrigeration requirements for liquefaction. Such cycles typically utilize combinations of single-component refrigeration systems using propane or single chlorofluorocarbon refrigerants operated in combination with one or more mixed refrigerant (MR) systems. Well-known mixed refrigerants typically comprise light hydrocarbons and optionally nitrogen, and utilize compositions tailored to the temperature and pressure levels of specific process steps. Dual mixed refrigerant cycles also have been utilized in which the first mixed refrigerant provides initial cooling at warmer temperatures and the second refrigerant provides further cooling at cooler temperatures.
U.S. Pat. No. 3,763,658 discloses a LNG production system which employs a first propane refrigeration circuit which precools a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed.
An alternative LNG production system, described in U.S. Pat. No. 4,065,278, uses a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature below the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This process differs from U.S. Pat. No. 3,763,658 cited above in that the distillation of the feed for heavy component removal occurs at a temperature lower than that provided by the first refrigeration circuit, and a pressure substantially lower than the feed pressure.
U.S. Pat. No. 4,404,008 discloses a LNG production system which employs a first propane refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed using a valve and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid stream, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This prior art differs from U.S. Pat. No. 3,763,658 in that cooling and partial condensation of the mixed refrigerant of the second refrigeration circuit occurs between compression stages. The resulting liquid is then recombined with the resulting vapor stream at a temperature warmer than the lowest temperature of the first refrigeration circuit, and the combined mixed refrigerant stream is then further cooled by the first refrigeration circuit.
An alternative LNG production system is disclosed in U.S. Pat. No. 4,274,849 which system employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a temperature lower than the intermediate temperature of the liquid, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. In FIG. 7 of this reference, the vapor resulting from the separation of the second refrigerant after precooling is further cooled to a temperature lower than that provided by the first refrigeration circuit and separated into liquid and vapor streams.
U.S. Pat. No. 4,539,028 describes a LNG production system which employs a first mixed component refrigeration circuit to precool a second mixed component refrigeration circuit. After the final stage of precooling by the first refrigeration circuit, mixed refrigerant from the second refrigeration circuit is separated into liquid and vapor streams. The resulting liquid stream is subcooled to an intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration. The resulting vapor stream is liquefied, subcooled to a lower temperature than the intermediate temperature, flashed across a throttling valve, and vaporized to provide refrigeration and final cooling of the feed. This patent differs from that of U.S. Pat. No. 4,274,849 described above by the fact that the second refrigerant is vaporized at two different pressures to provide refrigeration.
The state of the art as defined above describes the vaporization of subcooled mixed refrigerant streams to provide refrigeration for natural gas liquefaction wherein the subcooling is provided by a portion of the refrigeration generated by flashing and vaporizing of the subcooled mixed refrigerant streams. Refrigeration for cooling the mixed refrigerant streams and the natural gas feed is provided by the vaporization of mixed refrigerant streams in a main heat exchange zone. Cooling of the mixed refrigerant vapor during and/or after compression is provided by a separate refrigerant such as propane.
Improved efficiency of gas liquefaction processes is highly desirable and is the prime objective of new cycles being developed in the gas liquefaction art. The objective of the present invention, as described below and defined by the claims which follow, is to improve liquefaction efficiency by providing an additional vaporizing refrigerant stream in the main heat exchange zone. Various embodiments are described for the application of this improved refrigeration step which enhance liquefaction efficiency.
The invention relates to a method for providing refrigeration for liquefying a feed gas which comprises:
(1) providing refrigeration from a first recirculating refrigeration circuit which provides refrigeration in a temperature range between a first temperature and a second temperature which is lower than the first temperature;
(2) providing refrigeration from a second recirculating refrigeration circuit in a temperature range between the second temperature and a third temperature which is lower than the second temperature, wherein the first refrigeration circuit provides refrigeration to the second refrigeration circuit in the temperature range between the first temperature and the second temperature;
(3) withdrawing a mixed refrigerant vapor from a main heat exchange zone in the second recirculating refrigeration circuit and compressing the mixed refrigerant vapor to a final highest pressure to yield a compressed mixed refrigerant vapor;
(4) partially condensing at least a portion of the mixed refrigerant vapor in the second recirculating refrigeration circuit and separating the resulting partially condensed mixed refrigerant into at least one liquid refrigerant stream and at least one vapor refrigerant stream; and
(5) subcooling the at least one liquid refrigerant stream to a temperature lower than the second temperature, reducing the pressure of the resulting subcooled liquid refrigerant stream, and vaporizing the resulting reduced-pressure refrigerant stream to provide at least a portion of the refrigeration for liquefying the feed gas between the second temperature and the third temperature.
The step of partially condensing the compressed mixed refrigerant vapor is effected at a pressure essentially equal to the final highest pressure.
The refrigeration for liquefying the feed gas between the second temperature and the third temperature can be provided by indirect heat exchange with a vaporizing mixed refrigerant in a main heat exchange zone. This vaporizing mixed refrigerant is provided by
(a) compressing the mixed refrigerant vapor to a first pressure;
(b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a first mixed refrigerant vapor fraction and a first mixed refrigerant liquid fraction;
(c) subcooling the first mixed refrigerant liquid fraction to provide a first subcooled mixed refrigerant liquid;
(d) reducing the pressure of the first subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide vaporizing mixed refrigerant for cooling and condensing the feed gas therein; and
(e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor for step (a).
At least a portion of the refrigeration for the subcooling in step (c) can be provided by the vaporizing of the reduced pressure mixed refrigerant in the main heat exchange zone in step (d). At least a portion of the refrigeration for the subcooling in (c) can be provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone. The one or more additional refrigerant streams can comprise a single component refrigerant or a multicomponent refrigerant.
The method can further comprise partially condensing and separating the first mixed refrigerant vapor fraction to yield a second mixed refrigerant vapor and a second mixed refrigerant liquid, subcooling the second mixed refrigerant liquid by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting subcooled second mixed refrigerant liquid, and vaporizing the resulting reduced pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
The method also can further comprise condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor, and vaporizing the resulting reduced-pressure mixed refrigerant stream in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
Typically, at least a portion of the refrigeration for the cooling and partial condensing in (b) can be provided by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone. At least one of the one or more additional refrigerant streams can comprise a single component refrigerant a multicomponent refrigerant.
A portion of the refrigeration for cooling the feed gas can be provided by indirect heat exchange with one or more additional refrigerant streams external of the main heat exchange zone. The one or more additional refrigerant streams can comprise a single component refrigerant or a multicomponent refrigerant.
The feed gas can comprise methane and one or more hydrocarbons heavier than methane, and in this case the method can further comprise:
(e) precooling the feed gas by indirect heat exchange with an additional refrigerant stream;
(f) introducing the resulting precooled feed gas into a scrub column with a lean scrub liquid enriched in hydrocarbons heavier than methane;
(g) withdrawing from the bottom of the scrub column a stream rich in hydrocarbons heavier than methane;
(h) withdrawing from the top of the scrub column an overhead stream containing methane and residual hydrocarbons heavier than methane;
(i) cooling the overhead stream in the main heat exchange zone to condense residual hydrocarbons heavier than methane;
(j) separating the resulting cooled overhead stream into a purified methane-enriched product and a stream enriched in hydrocarbons heavier than methane; and
(k) utilizing at least a portion of the stream enriched in hydrocarbons heavier than methane to provide the lean scrub liquid of (f).
The first mixed refrigerant vapor fraction can be compressed following separation in (b). The cooling and partially condensing of the resulting compressed first mixed refrigerant vapor in (b) can be effected by indirect heat exchange with a fluid at ambient temperature. A portion of the first mixed refrigerant liquid can be mixed with the first pressurized mixed refrigerant vapor.
Optionally, at least a portion of the first mixed refrigerant vapor in (b) can be further cooled, partially condensed, and separated into an additional mixed refrigerant liquid which is combined with the first pressurized mixed refrigerant liquid. A portion of the refrigeration for cooling and partially condensing the first mixed refrigerant vapor fraction can be provided by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone.
The first pressurized mixed refrigerant liquid after subcooling can be vaporized in the main heat exchange zone at a first pressure and the second pressurized mixed refrigerant liquid after subcooling can be vaporized in the main heat exchange zone at a second pressure. The method can further comprise condensing and subcooling the second mixed refrigerant vapor by indirect heat exchange with vaporizing mixed refrigerant in the main heat exchange zone, reducing the pressure of the resulting condensed and subcooled second mixed refrigerant vapor to the second pressure, and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide additional vaporizing mixed refrigerant therein.
The operation of the second recirculating refrigeration circuit can include
(a) compressing the mixed refrigerant vapor to a first pressure;
(b) cooling, partially condensing, and separating the resulting compressed refrigerant vapor to yield a mixed refrigerant vapor fraction and a mixed refrigerant liquid fraction;
(c) subcooling the mixed refrigerant liquid fraction to provide a subcooled mixed refrigerant liquid;
(d) reducing the pressure of the subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure mixed refrigerant liquid in the main heat exchange zone to provide one of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein; and
(e) withdrawing a vaporized mixed refrigerant stream from the main heat exchange zone to provide at least a portion of the mixed refrigerant vapor in (a).
The refrigeration for subcooling the mixed refrigerant liquid fraction can be provided in part by indirect heat exchange with the resulting vaporizing reduced pressure refrigerant liquid in the main heat exchange zone and in part by indirect heat exchange with one or more portions of an additional refrigerant external to the main heat exchange zone.
The operation of the second recirculating refrigeration circuit can further comprise
(f) condensing and subcooling the mixed refrigerant vapor fraction to provide an additional subcooled mixed refrigerant liquid; and
(g) reducing the pressure of the additional subcooled mixed refrigerant liquid and vaporizing the resulting reduced pressure liquid in the main heat exchange zone to provide another of the vaporizing mixed refrigerant streams for cooling and condensing the feed gas therein.
The refrigeration for condensing and subcooling the additional mixed refrigerant vapor can be provided in part by indirect heat exchange with the resulting vaporizing reduced pressure liquid in the main heat exchange zone and in part by indirect heat exchange with one or more additional refrigerant streams external to the main heat exchange zone.